In the microelectronics industry, in particular, thin films of metals are often deposited on insulating substrates, where the insulators are typically organic polymers, such as polyimide and other types of plastics and polyesters. In current microelectronics technology, polymer materials and their interfaces (such as those with metals) play a very important role. With the trend toward higher speed semiconductor devices and higher density circuits on semiconductor chips, signal delays on chips have become comparable to or shorter than those associated with the packaging of the chip-i.e., wiring between chips located on the same or different substrates, cards, boards, etc. As the packaging delays have become comparable to chip delays, new packaging schemes utilizing thin film metal interconnections electrically isolated by polymeric materials have been adopted. These techniques have been motiviated by cost and reliability considerations, as well as by the required compatability with manufacturing automation.
The use of polymers offers a number of intrinsic advantages in microelectronics applications. Polymers are low in cost and relatively easy to handle, and in addition offer a variety of desireable properties. These properties include flexibility, planarizability and stability. The polymers generally can be tailored to match the needs of the particular application by using polymer chemistry and engineering. Additionally, they can be processed readily in a variety of ways, such as plasma etching, reactive ion etching, and laser ablation. Relatively high temperature stability is especially attractive for those applications where annealing steps are involved in the processing. For example, polyimide is a widely used insulator since it is stable to approximately 400.degree. C.
The miniaturization of semiconductor devices and other microelectronic devices requires a further reduction of device dimensions and multilevel wiring schemes on the chip. The insulators used between these wiring levels have to fulfill a number of requirements, which are generally well met by various types of organic insulators, including polymers. These insulators must be applied with good thickness uniformity, have excellent electrical integrity and thermal stability, and exhibit good adhesion to the metal layers they keep separated. This last problem-that of metal/polymer adhesion-has not been entirely successfully addressed by the industry.
Many metals bond very well to polymer-type substrates, such as polyimide. The metals which generally bond well to provide good adhesion are those which form relatively strong chemical bonds to the polyimide atomic constituents (carbon, oxygen, nitrogen, and hydrogen). However, many metals are very weakly interacting with the microscopic constituents of these types of substrates and do not bond well. For these metals, the adhesion problem described previously has not been solved.
Various treatments have been utilized in the prior art to enhance adhesion between metals and polymers. These techniques include roughening of the polymer surface and chemical pre-sensitizing prior to the deposition of a metal, as described in U.S. Pat. No. 3,881,049.
Another technique for depositing copper on polyimide was described by M. Terasawa et al at the International Symposium on Hybrid Manufacturing (ISHM), Oct. 31-Nov. 2, 1983, Philadelphia. In this technique, copper and aluminum are deposited by ion-plating using electron beam deposition in order to enhance the conductor adhesion. Chromium is used as a substrate metal to provide enhanced adhesion between the copper or aluminum and the Cr-coated substrate.
The use of heating to promote adhesion between metals and polymers is well known in the art. For example, reference is made to U.S. Pat. No. 4,152,195, in which a polyimide resin layer is partially cured by heating to approximately 200.degree. C. for 20 minutes and then to approximately 275.degree. C. for 30 minutes, prior to metal deposition. After a metal is vapor deposited on the partially cured polyimide, the entire structure is fully cured by heating to 350.degree. C. This reference also describes other techniques to improve the adherence of aluminum layers to polyimide surfaces. These prior techniques include roughening of the polyimide surface by sputtering, by chemical solutions, or by the presence of oxygen atoms and electrical discharges which partly burn the polyimide surfaces. Fully curing the polyimide prior to metal deposition is also described in this reference.
Another reference which describes improved adhesion between copper and polyimide is U.S. Pat. No. 4,386,116. In this reference, the polyimide is heated to a temperature of approximately 360.degree.-380.degree. C. during the evaporation of chrome-copper. A critical substrate temperature of 360.degree.-380.degree. C. was required to produce copper-copper interface vias with good electrical conductivity and to provide optimum polyimide stability. It was suggested that a chemical reaction between copper and nitrogen in the polyimide provided the necessary adhesion.
In the prior art, it has been the situation that the polyimide substrate is heated to a temperature as high as possible, that is, to a temperature approximating the c-curing temperature of the polyimide. For example, chromium is generally deposited on polyimide at high temperatures, such as approximately 360.degree.-380.degree. C., in order to provide enhanced adhesion. Thus, the teaching of the prior art has been to use temperatures as high as possible in order to promote enhanced bonding of a metal to a polymer substrate, within the limits of the temperature stability of the polyimide. This approach has been used both for metals which chemically bond (i.e., strongly interact) with the polymer substrate as well as those which are only very weakly interacting with the substrate.
In the course of applicants' experimentation, it has been discovered that, for those metals which have only very weak bonding with polymer substrates, a critical range of substrate temperatures and deposition rates exists in which adhesion can be maximized. Applicants have discovered that intermixing of the depositing metal atoms and the polymer substrate occurs at the interface as a result of heating. This intermixing is increased when the metal atoms have sufficient time to diffuse into the polymer substrate before combining with further incoming metal atoms. The time required for diffusion into the polymer substrate is shorter at higher substrate temperatures. Thus, the substrate temperature, deposition rate, and annealing time are adjusted so that diffusion of the metal atoms (i.e., intermixing) into the polymer substrate occurs prior to the combination of the metal atoms with other metal atoms. If the deposition rate of the arriving metal atoms is small with respect to the diffusion rate of metal atoms into the substrate, significant intermixing will occur. However, applicants have discovered that the amount of intermixing has to be optimized in order to maximize adhesion. It has also been found that, contrary to the teaching of the prior art, a substrate temperature less than the temperature at which the polymer cures produces optimum intermixing and, therefore, maximum adhesion. For example, in the case of copper or nickel deposition onto polyimide, a substrate temperature of approximately 240.degree.-280.degree. C. provides maximum adhesion. Generally, the optimal temperature range is approximately (0.6-0.8) T.sub.c, where T.sub.c is the curing temperature of the polymer substrate.
Accordingly, it is a primary object of the present invention to provide a technique for depositing selected metals onto polymer substrates with improved adhesion, where the metals are those which only weakly intrinsically bond to the atomic constituents of the polymer substrate.
It is another object of the present invention to provide improved polymer substrate-metal structures which exhibit improved adhesion, where the metal overlayers are those which normally only weakly bond to the substrates.
It is another object of the present invention to provide improved adhesion of Ni, Cu, and Al to polymer substrates.
It is another object of the present invention to maximize the adhesion between intrinsically weakly bonding metals and organic substrates, which technique can be readily adapted into manufacturing lines.
It is another object of this invention to provide improved packaging techniques and microelectronic structures having enhanced adhesion at metal-polymer interfaces.
It is another object of this invention to provide structures including interfaces between polymers and metals that intrinsically bond very weakly to the polymer constituents, where enhanced adhesion is produced between the metals and the polymers.